Planning for the final assault on Raqqa, the capital of the Islamic State’s caliphate, had been grinding on for more than seven months. There had been dozens of meetings of President Barack Obama’s top national security team, scores of draft battle plans and hundreds of hours of anguished, late-night debates.

There were no good options, but Obama’s top foreign policy advisers were convinced that they had finally settled on an approach that could work — arming Kurdish fighters in northern Syria, current and former U.S. officials said. There was just one problem: The Obama team had deliberated for so long that there was no time left to pull the trigger.

So on Jan. 17, just three days before the transfer of power, Obama directed his national security adviser to hand over to the Trump team a paper detailing the plan to arm the Kurds, including talking points that President Trump could use to explain the move to Turkey’s president, who officials knew would be furious. The Turks viewed the Kurdish fighters as terrorists and their No. 1 enemy.